The present invention relates generally to firing circuits for thyristor power conversion systems and more particularly to an improved circuit for controlling the operation of the power conversion system which supplies an electrical load such as an AC motor from a polyphase alternating current source.
Many circuits and systems exist for controlling the conductivity of controlled rectifiers utilized in various types of converters for supplying electrical power to a load from a polyphase alternating current (AC) source. The type of rectifier used will, of course, control to some degree the type of control utilized, but by far the most common controlled rectifier used today is a thyristor of the silicon controlled rectifier type. The thyristor becomes conductive with the simultaneous application of a forward bias voltage and a signal applied to its gate electrode and remains conductive until the anode current falls below the value required to hold the thyristor in the conductive state.
In any polyphase converter system, each phase carries all the current for a portion of the time. For example, in a three phase system, each phase carries all the current for one third of the time with the transfer from phase to phase being accomplished by a well known process commonly referred to as commutation. When the alternating current source applying the thyristor bridge has significant inductance, the commutation of the current from phase to phase takes both voltage and time and accordingly, during commutation process, two phases become shorted for the time it takes to commutate the line current from the outgoing phase to the oncoming phase. The duration of this short circuit is related to the commutation inductance (the inductance of the source supplying the bridge) and the amplitude of the current being commutated. As a result the nominally sinusoidal AC line-to-line voltage becomes corrupted by what is commonly referred to as commutation notches which are periods of zero voltage each time a new thyristor is fired or six times the fundamental line frequency for a conventional six pulse thyristor bridge.
In a phase control system, whether it be an analog or digital control system, the AC terminal voltage is a prime feedback signal employed for the thyristor bridge control. Typically, the AC and DC terminal voltages, referenced to a fictitious neutral, are coupled from the high voltage thyristor bridge through a high impedance resistor attenuator string into differential amplifiers in the control circuitry and to other related circuitry to further derive various signals for a variety of purposes such as thyristor state detectors and voltage regulation. The principal use of the terminal voltage, however, is in a phase-locked loop firing control circuit wherein synchronizing signals are generated from processed line voltages typically involving the integration of the AC line-to-line voltage. In such applications, the zero voltage commutation notches appearing in the line-to-line voltages generate flat spots in the integrated output voltage with the positioning of the notches being dependent on the actual firing angle and their duration which is dependent on the line current and the inductance in the commutation path. Typically, the zero crossings of the integrated line-to-line voltages are determined by comparators and the digital comparison for each phase are combined to form a six times line frequency synchronizing pulse train. Since the aforementioned flat spots can occur at the integrated voltage zero crossings, the stability of the phase lock loop can be undesirably affected. To overcome this undesirable occurrence, known prior art practice has resorted to reconstructing the line-to-line voltage waveform by, in effect, filling up the commutation notches by a method of superposition involving summing the corrupted line-to-line voltage with a signal proportional to the commutating inductance multiplied by the derivative of a fictitious "delta" current which is derived by taking the difference between actual line currents in a well known manner and as more fully described later. The resultant or composite sine wave voltage is thereafter integrated and utilized as a primary feedback control signal for synchronizing either a fixed frequency source side converter or variable frequency load side converter or both.
With respect to a load side converter for a three phase (3.phi.) AC motor drive, the line-to-line voltage comprises the back electromotive force (emf) which when integrated approximates a pseudo motor flux wave which is expressed in units of volt-seconds. Control of the load side converter furthermore is usually based upon the desire to fire (i.e. to render conductive) the converter as late as possible, i.e. at a power factor angle just sufficiently leading to provide the volt-seconds necessary to safely commutate the outgoing thyristor, according to the expression:
E.DELTA.t=L .DELTA.i; wherein: PA1 E=source voltage, PA1 .DELTA.t=commutation time, PA1 L=commutation inductance, PA1 .DELTA.i=current being commutated.
Accordingly, accurate "flux wave" zero crossings and maximum amplitude information are required in the load side converter to maintain stable and accurate phase locked loop operation, commutation and speed regulation. While filling up the commutation notches in the feedback signal by a summation with the appropriate Ldi/dt signal has heretofore been utilized to reconstruct the voltage signal it requires a differentiation of the motor currents. The network required to implement the differentiation, however, must be followed by a higher frequency lag circuit to eliminate noise in the control loop and thus unduly complicates the system. It is to this problem that the present invention is directed.
It should also be pointed out that whereas motor control systems employing controllable rectifiers hereinafter referred to as thyristors, have been implemented using analog control devices, more recent attention has been directed to digital type of control systems, one typical example of which is disclosed in U.S. Pat. No. 3,601,674 entitled, "Control System For Firing SCR'S In Power Conversion Apparatus", issued to John A. Joslyn and Albert F. Koch on Aug. 24, 1971 and assigned to the assignee of the present invention. In this patent, a digital control system is disclosed for controlling the flow of power through thyristors from a polyphase AC source to a load. The system disclosed includes a firing circuit for each phase, wherein each firing circuit comprises a reversible counter and a digital comparator. Phase detection logic is incorporated which examines the three phases of the AC source to synchronously initiate a control interval for an appropriate rectifier by presetting a predetermined positive or negative digital number into a reversible counter associated with each phase. The reversible counter then counts down if the preset number is positive or up if the preset number is negative during the control interval. During counting, a digital speed error signal, derived from a previous comparison of a digital command with a digital feedback signal indicative of motor speed is continuously compared with the contents of the reversible counter by the digital comparator. When the error exceeds the contents of the reversible counter, a firing pulse is generated and supplied to a positively or negatively poled rectifier, firing the respective poled rectifier in accordance with the positive and negative number.
Still more recently a programmed digital computer has been employed to control the firing of the thyristors through a programmed interrupt operational procedure. Such a system has been described in two publications, first by R. D. Jackson and R. D. Weatherby entitled, "Direct Digital Control Of Thyristor Converters", in the IFAC Symposium of Control and Power Electronics and Electrical Drives, dated October, 1974 and secondly in a publication by F. Fallside and R. D. Jackson entitled, "Direct Digital Control of Thyristor Amplifiers" appearing in the Proceedings of the Institution of Electrical Engineers-Control and Science, Volume 116, No. 5, pp. 873-878, May, 1969.